1. Field of the Invention
The present invention relates generally to nuclear reactor internals and more specifically to apparatus for maintaining the alignment of the nuclear reactor internals.
2. Description of the Prior Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat-exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internals structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. The primary side is also connected to auxiliary circuits, including a circuit for volumetric and chemical monitoring of the pressurized water. The auxiliary circuit, which is arranged branching from the primary circuit, makes it possible to maintain the quantity of water in the primary circuit by replenishing, when required, with measured quantities of water, and to monitor the chemical properties of the coolant water, particularly its content of boric acid, which is important to the operation of the reactor.
The average temperature of the core components during full power reactor operation is approximately 580 F (304° C.). Periodically, it is necessary to shut down the reactor system for maintenance and to gain access to the interior side of the pressure vessel. During such an outage, the internal components of the pressure vessel can cool to a temperature of approximately 50° F. (10° C.). The internal components of the pressure vessel typically consist of upper and lower internals. The upper internals include a control rod guide tube assembly, support columns, conduits for instrumentation which enter the reactor vessel through the closure head, and a fuel assembly alignment structure, referred to as the upper core plate. The lower internals include a core support structure referred to as the core barrel, a core shroud that sits inside the core barrel and converts the circular interior of the barrel to a stepped pattern that substantially corresponds to the perimeter profile of the fuel assemblies that constitute the core supported between a lower core support plate and the upper core plate. As an alternate to the shroud, a bolted baffle former structure consisting of machined horizontal former and vertical baffle plates, has been employed. It is particularly important to maintain a tight alignment of the reactor internals upper core plate and a top plate of the shroud with the control rod drive mechanisms to assure that the control rods can properly scram; i.e., drop into the core, when necessary. This is particularly challenging when one considers the thermal expansion and contraction that has to be accommodated through power ramp-up and cool down sequences, where temperatures can vary between 50° F. (10° C.) and 580° F. (304° C.)
In conventional designs, lateral alignment of the upper internals components was accomplished with a series of single pins located around the circumference of the core barrel. The upper core plate alignment pins fit in notches in the upper core plate and locate the upper core plate laterally with respect to the lower internals assembly. The pins must laterally support the upper core plate so that the plate is free to expand radially and move axially during differential thermal expansions between the upper internals and the core barrel. FIG. 1 is a simplified cross-section of such a conventional reactor design. A pressure vessel (10) is shown enclosing a core barrel (32) with a thermal shield (15) interposed in between. Some plants have neutron pads in lieu of the thermal shield. The core barrel (32) surrounds the core (14) which is held in position by an upper core plate (40). The upper core plate (40) is aligned by the alignment pins (19) which extend through the core barrel (32) into notches (21) in the upper core plate (40). The notches (21) permit the core barrel to grow with thermal expansion at a greater rate than the upper core plate (40) during start up without compromising the lateral position of the upper core plate (40). The installation sequence of the core shroud (17) in new advanced passive plant designs requires a modified design that will prevent lateral movement of the upper core plate and the core shroud while enabling thermal growth and differential expansion between both the shroud and the upper core plate and the core barrel, while maintaining rotational stability.
New passive nuclear plant designs employ a core shroud assembly that is primarily a welded structure. The typical manufacturing process is to assemble the core shroud fully outside the lower internals core barrel. After assembly, the core shroud assembly is lowered into the lower internals. In this arrangement, it is not possible to have protruding alignment pins (19) to engage the upper internal's core plate. The protruding alignment pins would interfere with the core shroud bottom plate, core shroud panel reinforcements, etc., during insertion within the core barrel. Therefore, an alternate alignment feature was identified to accommodate the advanced passive plant internals design.
To align the core shroud and upper internals this alternate alignment feature comprises four alignment plates, secured to the lower internals core barrel with a set of bolts and dowel pins. The alignment plates are installed after installation of the core shroud assembly within the lower internals. Custom fit inserts are used to align both the lower and upper internals with each other via the alignment plates. However, the installation of the alignment plates involves machining four slots, or grooves, in the inside diameter of the core barrel; one groove is required for each alignment plate. The grooves are required to verify set up of the alignment plates prior to installation of the core shroud assembly. The alignment plates are installed in the lower internals after installation of the core shroud assembly. To provide clearance to slide the alignment plate into the machined groove in the core barrel inside diameter, the core shroud top plate slot depth is increased 0.750″ (1.905 centimeters), as compared to nominal value. This 0.750″ (1.905 centimeter) increase occurs at a location adjacent to one of the more limiting core shroud top plate ligaments. After securing each alignment plate with dowel pins and six bolts, the 0.750″ (1.905 centimeter) gap between the alignment plate and the core shroud top plate is filled by installation of a customized insert. In view of the installation sequence for installing the alignment plates, it's likely that it may be difficult to remove the core shroud assembly, should there be a need during the 60 year design life of the advance passive plant designs. Accordingly, an alternate design is desired that would further facilitate manufacture, installation and removal of the core internals while maintaining rotational alignment between the core shroud and the upper core plate.
It is an object of this invention to provide such a further improvement that will additionally facilitate manufacture, satisfy the alignment requirements and permit later removal of the core shroud assembly in tact.